The readability of the information displayed by display devices such as liquid crystal display cells or organic light-emitting diode display devices is highly dependent on ambient lighting conditions. With some display devices, the displayed information can be read in good conditions in a lit environment, but is difficult to read in a dark environment. Conversely, other categories of display devices provide a good quality display in twilight or darkness, but are difficult to read in broad daylight.
By way of example, let us consider transflective liquid crystal display cells, that is to say liquid crystal display cells capable of displaying information that will be visible in daytime by exploiting the phenomenon of ambient light reflection, and which will also be visible at night by transmission using a backlight device. Such transflective liquid crystal display cells are optimised to provide the best possible reflection of sunlight and thus to ensure good readability of the displayed information in bright ambient conditions. However, in order for such transflective liquid crystal display cells to be capable of the best possible reflection of sunlight, their transmission efficiency is greatly restricted. Thus, when the backlight device is activated to allow the displayed information to be read in twilight, most of the light emitted by the backlight device is lost in absorption phenomena. Energy efficiency in this situation is therefore poor. Further, the optical qualities of the information displayed by the liquid crystal cell are highly dependent on the viewing angle.
As regards emissive display devices, such as organic light-emitting diode display devices, these devices have superior optical qualities to those of liquid crystal display cells, since optical qualities, such as luminance and colour, are not dependent on the viewing angle. Nonetheless, these high quality emissive display devices do not permit a reflective mode of operation. The information displayed thereby is thus very readable in twilight or darkness, but becomes difficult to read once observed outdoors. To overcome this problem, it is possible to increase the amount of current supplied to emissive display devices in order to ensure a minimum level of readability. However, even in normal conditions of use, these emissive display devices use more power than a reflective liquid crystal cell. Their electrical power consumption is therefore such that it is difficult to envisage keeping them permanently turned on, in particular when they are incorporated in a portable object of small dimensions, such as a wristwatch, whose only source of energy is a battery which is usually required to last for more than one year.